1. Field of the Invention
The present invention relates to a phase shift mask and a technique relevant thereto, and more specifically, it relates to a shading member provided on a phase shift mask employed for a step of exposing a semiconductor device.
2. Description of the Background Art
FIG. 10 schematically illustrates the structure of an exposure unit employed for a conventional step of exposing a semiconductor device. In this exposure unit, an exposure light source 21 emits exposure light 22 having a prescribed exposure wavelength. The emitted exposure light 22 is transmitted through condenser lenses 23a and 23b, and the exposure area thereof is adjusted by a blind 24.
The exposure light 22 having the adjusted exposure area is transmitted through condenser lenses 23c and 23d, and thereafter transmitted through a phase shift mask (including a reticle mask throughout the specification) 25 formed with a prescribed pattern layout.
The exposure light 22 transmitted through the phase shift mask 25 is adjusted to a prescribed exposure area by projection lenses 26a and 26b and an NA diaphragm 28, and thereafter exposes a semiconductor substrate 14 placed on a stage 30.
The semiconductor substrate 14 is sequentially formed with exposed areas 31 and 32 due to X-Y directional movement of the stage 30 (step-and-repeat system), and the pattern layout of the phase shift mask 25 is exposed over the entire surface of the semiconductor substrate 14.
The pattern layout of the phase shift mask 25 is roughly classified into an LSI circuit pattern area 27 and a non-exposed area 20. The LSI circuit pattern area 27 is formed with a pattern structure by a phase shift method capable of exposing a fine pattern, in order to satisfy the recent requirement for refinement of semiconductor devices.
With reference to FIGS. 11(A) and 11(B), the basic principle of a halftone phase shift method, representing the phase shift method, is now described. FIG. 11(A) is a sectional view of a halftone phase shift mask, and FIG. 11(B) illustrates the intensity of exposure light transmitted through the halftone phase shift mask on a semiconductor substrate.
Referring to FIG. 11(A), a transmission area 34 and a phase shifter area 33 are formed on a transparent substrate 4 in this halftone phase shift mask. Part of the exposure light transmitted through the phase shifter area 33 is so adjusted that the phase thereof is converted by 180xc2x0 and the transmittance is about 2 to 40% with respect to that transmitted through the transmission area 34. Thus, in the exposure light transmitted through the halftone phase shift mask having the structure shown in FIG. 11(A), light components 180xc2x0 0 out of phase overlap and cancel with each other in the vicinity of the boundary between the phase shifter area 33 and the transmission area 34. Consequently, an area having light intensity of zero is formed in the vicinity of the boundary between the phase shifter area 33 and the transmission area 33, as shown on a light intensity curve 35 in FIG. 11(b).
The intensity of the part of the exposure light transmitted through the phase shifter area 33 is adjusted to a level (line L1 in FIG. 11(B)) not exposing a target film provided on the semiconductor substrate, by adjusting the transmittance of the phase shifter area 33. Thus, exposure of a fine pattern corresponding to the transmission area 34 provided on the halftone phase shift mask is enabled.
A halftone phase shift mask employing this halftone phase shift method is now described with reference to FIGS. 12 and 13. FIG. 12 is a plan view of the halftone phase shift mask, and FIG. 13 is a sectional view taken along the line X-Xxe2x80x2 in FIG. 12.
This halftone phase shift mask includes an LSI circuit pattern area 2, a strip-shaped shading zone area 3 provided to enclose the LSI circuit pattern area 2, and a non-exposure area 13 provided to enclose the shading zone area 3.
The LSI circuit pattern area 2 is formed with an LSI circuit pattern employing the aforementioned halftone phase shift method. The shading zone area 3 is provided for shading light leaking through a clearance defined between the LSI circuit pattern area 2 and the blind 28 described with reference to FIG. 10.
The principle of shading with the shading zone area 3 is now described with reference to FIGS. 14 and 15. FIG. 14 is an enlarged plan view of a region enclosed with a circle A in FIG. 12, and FIG. 15 is a sectional view taken along the line X-Xxe2x80x2 in FIG. 14.
The shading zone area 3, formed by shading parts 3a and Hall patterns 3b of the same material as a phase shifter area (not shown) forming the LSI circuit pattern area 2 employing the halftone phase shift method, can reduce the intensity of exposure light transmitted therethrough to a level not exposing a target film due to the function/effect of the halftone phase shift method. The size of each Hall pattern 3b is set smaller than that of the pattern of the resolution limit of the exposure unit so that each side is several xcexcm, for example. Therefore, the exposure light transmitted through the shading zone area 3 forms no image.
Consequently, the shading zone area 3 can substantially prevent the exposure light transmitted therethrough from leakage, for serving as a shading film.
The semiconductor substrate 14 is sequentially formed with the exposed areas 31 and 32 by the step-and-repeat system as described with reference to FIG. 10, so that a pattern 17 of the phase shift mask 25 is exposed over the entire surface of the semiconductor substrate 14 as shown in FIG. 16.
As shown in FIG. 17, areas E irradiated with the exposure light transmitted through the shading zone area 3 are formed around a pattern 17a. When the pattern 17 of the phase shift mask 25 is sequentially exposed in the step-and-repeat system in order of the pattern 17a, a pattern 17b, a pattern 17c and a pattern 17d, for example, there are formed areas 15a where the areas E overlap with adjacent patterns and areas 15b where the areas E of four patterns overlap with each other.
In each area 15a, another area E is exposed (single exposure) on the original pattern exposure. In each area 15b, other three areas E are exposed (triple exposure) on the original exposure. Thus, particularly the area 15b is irradiated with exposure light having intensity exposing the target film as a result, to exert influence on the original pattern 17.
Considering this in view of the halftone phase shift mask, it follows that the halftone phase shift mask latently has areas 18 causing single exposure on linear portions of the shading zone area 3 and areas 19 causing triple exposure on corner portions of the shading zone area 3, as shown in FIG. 18. In this regard, there has been proposed a countermeasure of providing shading members 36 preventing transmission of exposure light on the areas 19 causing triple exposure, to be enclosed with corner portions of the shading zone area 3 as shown in FIG. 19.
However, this countermeasure is unpractical due to remarkable influence on the size of the fabricated semiconductor device.
If the shading members 36 are not formed on the shading zone area 3 but separately prepared and bonded with an adhesive or the like, the shading members 36 come into contact with both of a glass substrate 4 and the shading zone area 3 having different thermal expansion coefficients. When the glass substrate 4 and the shading zone area 3 are thermally expanded respectively, therefore, the shading members 36 are disadvantageously warped.
When the shading members 36 are provided to be enclosed with the corner portions of the shading zone area 3, a shading zone of a different size must be prepared for every phase shift mask since the size of the LSI circuit pattern area 2 varies with the phase shift mask.
While the Hall patterns 3b of the shading zone area 3 may conceivably be reduced in size, the resolution limit of the exposure unit is reduced following the recent development of the exposure unit and hence it is difficult to form Hall patterns in a size smaller than the resolution limit.
As shown in FIGS. 20 and 21, a shading zone 41 having transmittance of 0% may be provided on the shading zone area 3 by film formation. In this case, however, the fabrication steps are complicated and the yield of the phase shift mask is reduced, to disadvantageously increase the fabrication cost for the phase shift mask. FIG. 21 is a sectional view taken along the line X-Xxe2x80x2 in FIG. 20.
An object of the present invention is to provide a phase shift mask, particularly a phase shift mask capable of readily implementing a structure exerting no bad influence on adjacent exposed areas when exposing a semiconductor substrate.
Another object of the present invention is to provide a semiconductor device exposed with the aforementioned phase shift mask.
A phase shift mask according to the present invention comprises a circuit pattern area having a rectangular plane shape and a strip-shaped shading zone area provided to enclose the circuit pattern area on a substrate, and a shading member is mounted on the upper side of a corner portion of the circuit pattern area enclosing a corner portion of the circuit pattern area having a rectangular shape.
According to this phase shift mask, it is possible to prevent triple exposure on an area generally causing triple exposure, precisely form the pattern of a semiconductor device, and finely fabricate a semiconductor device in high quality also when sequentially exposing a semiconductor substrate to be exposed in a step-and-repeat system.
A separately formed shading member is mounted on the upper side of the corner portion of the shading zone area causing triple exposure. Thus, the mounted shading member is smaller in size than a shading member mounted to enclose the overall LSI circuit pattern area, for example. Therefore, tension resulting from difference between the thermal expansion coefficients of a glass substrate and the shading member mounted thereon is so small that a reticle is hardly warped and the shading member is hardly separated.
The size of the corner portion of the shading zone area is constant regardless of the size of the LSI circuit pattern area, and hence the shading member can be employed regardless of the size of the phase shift mask. Thus, the cost for the shading member and that for the phase shift mask can be reduced.
Preferably, the shading member is so provided that light transmittance is gradually increased as separated from the corner portion of the circuit pattern area.
Preferably, a material having such a property that transmittance is increased as the thickness thereof is reduced, and the shading member is so provided that the thickness thereof is gradually increased as separated from the corner portion of the circuit pattern area.
Preferably, the shading member is so provided that a surface opposite to the substrate is gradually separated from a surface closer to the substrate as separated from the corner portion of the circuit pattern area.
Alternatively, the shading member is so provided that the surface closer to the substrate is gradually separated from the surface opposite to the substrate as separated from the corner portion of the circuit pattern area.
A semiconductor device according to the present invention is fabricated with a phase shift mask comprising a circuit pattern area having a rectangular plane shape and a strip-shaped shading zone area provided to enclose the circuit pattern area on a substrate, in which a shading member is mounted on the upper side of a corner portion of the circuit pattern area enclosing a corner portion of the circuit pattern area having a rectangular shape.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.